It is not a question of if, but rather a question of when the next large asteroid
will strike Earth. The readily apparent cratering record of the Moon and the
increasingly revealed cratering record of Earth attest to a long history of
cosmic collisions. That history continues today, as dramatically shown by the
splash of comet Shoemaker-Levy 9 into Jupiter in 1994. The resulting Earth-sized
blemishes on the tops of Jupiters clouds attest to the wallop packed even
by relatively small impacting bodies. Small impacts on our planet, such as the
one that occurred over Tunguska, Siberia, in 1908, may happen on average once
per thousand years. A larger impact, one capable of severely affecting the climate
with potentially devastating effects for civilization, may occur on average
every million years or so. Much more rare, perhaps an average of every 100 million
years, are impacts posing the threat of mass extinctions.

There is both good news and bad news in the fact that geologic time scales describe
the frequency of major impacts. The good news is that no contemporary Geotimes
readers are likely to witness or suffer from a devastating cosmic impact during
their lifetimes. Thus we scientists and our current political leaders can be
happily complacent about asteroid impacts and very likely get away with it!
The bad news is that there are no guarantees. Just because an event is unlikely
doesnt mean it wont happen today, next week, next year, or any time
in this century. If we choose to ignore the asteroid issue, we are living dangerously
and passing the problem down to future generations.

Thus from a standpoint of prudence or inevitability, there are asteroids in
our future. Current technology now has the capability to transform the uncertainties
and probabilities of the asteroid problem into deterministic answers.
We can discover and track the largest asteroids near the Earth and determine
whether any are on a collision course in the coming centuries.

Thanks largely to the pioneering push by geologist Gene Shoemaker, NASA adopted
the current Spaceguard Goal to discover 90 percent of the near-Earth
asteroids larger than 1 kilometer by the year 2008. (One kilometer is the estimated
size capable of causing major climatic effects.) So far, about one-half of the
estimated 1,100 of these objects have been discovered, occasionally creating
sensational headlines when preliminary orbits show small (but still existing)
probabilities for a collision during a future pass by Earth.

Fortunately, further measurements allowing refined orbits have, to date, removed
all substantial threats from all known objects. With each discovery and orbit
determination ruling out any foreseeable hazard, we are gaining assurance that
the odds remain in our favor for not suffering a global impact catastrophe in
our lifetimes.

While the risk of a global- or civilization-threatening impact is now being
retired, the risk remains for the more frequent smaller-scale impacts capable
of causing local or regional damage. A cost/benefit analysis recently completed
by a NASA-appointed team recommends pushing the survey limits well below 1 kilometer,
down to a size of 140 meters. This study team finds that dropping the discovery
goals down to this size reduces the total asteroid risk (over all sizes) by
90 percent. Achieving this goal is cost effective when considering the potential
worldwide casualties from both land impacts and tsunami-generating ocean impacts.
One or more dedicated large telescopes (with apertures of 4 meters or more)
operating for several decades could complete the job. Completing the census
substantially faster could require performing the survey from space.

Fortunately, the most likely outcome of current and expanded surveys is that
no sizeable object will be found to be on a collision course with Earth over
the next century. In essence, we are buying cosmic insurance in the form of
knowledge that our near-term future is safe. Unfortunately we can be certain
that surveys will yield false positives, with asteroids impacting the media
but not the actual Earth. Two types of false positives will occur: Type I is
already a familiar scenario, in which a preliminary orbit shows a future close
encounter with Earth, where the normal measurement uncertainties initially do
not rule out a collision; Type II is where an object is discovered to be on
a definitive collision course, but the size of the object is too small to penetrate
the atmosphere intact and cause any substantive harm.

A key step astronomers must take to minimize media frenzy over false positives
is to organize a central information source, similar to the National Hurricane
Center. The role of this center will be to convey with a single authoritative
voice the consensus findings of international experts regarding whether any
asteroid close encounter merits public or governmental concern. Continuing with
the present situation of multiple information sources, where even just a subtle
difference becomes polarized by the media, serves only to erode the credibility
of professional astronomers.

If we allow ourselves to think the unthinkable, where a sizable object is discovered
to be on a definitive collision course, for example 30 years from now, what
would we do? In spite of its lack of Hollywood appeal, deflection rather than
destruction is the best approach. But pushing on an asteroid may be extraordinarily
tricky, depending on whether the body is intact or severely fractured.
Some models for asteroid interiors even suggest they may be rubble piles,
reassembled blocks having no coherency between them, held together only by their
mutual gravity. Slow and gentle propulsion, such as that provided by an ion
drive or harnessed from solar radiation pressure, holds the best promise. Yet
for these slow and gentle schemes to work, time must be on our side. The only
way to put time on our side is for the surveys to move forward now.

Our next step, in concert with surveys, is to understand what these objects
are: Know thy enemy. Here is where scientists, especially geologists
and geophysicists, have their role. The questions we as scientists ask 
seeking to understand the compositions and internal structures of these space
mountains  are the same questions that must be answered in order
to achieve effective mitigation.

While we might relish the idea of geology field trips to these worlds, neither
the science alone nor the combination with the impact hazard concern justifies
the cost of sending humans (rather than robotic spacecraft) to do the job. My
view only changes if asteroids become a human destination as part of a stepping
stone strategy for operationally testing people and equipment with the
ultimate goal of missions to Mars. Many near-Earth asteroids provide destinations
for six-month to one-year missions in interplanetary space, shorter than the
risk of multi-year missions to Mars. Resource utilization may be another driver
for going to the asteroids, where low launch costs and economic viability are
essential for this future path. Completed surveys will reveal accessible and
promising destinations, making our asteroid future more likely filled with friends
than foes.

Binzel is a professor of planetary science at the Massachusetts Institute of
Technology in Cambridge. He is the inventor of the Torino Scale for public communication
of the asteroid impact hazard.

Opinions and conclusions expressed in Comment by the authors are their own
and not necessarily those of AGI, its staff or its member societies.